Methods of forming an irradiation field in a particle beam treatment system are approximately classified into a broad-beam irradiation method of enlarging a beam by the use of scattering substances and concurrently irradiating the whole affected part of a patient as an irradiation target with the enlarged beam and a scanning irradiation method (scanning method) of directly scanning and irradiating an affected part with a narrow beam by the use of an electromagnet. In any case, since the position and the angle (slope) of a charged particle beam emitted from an accelerator are not stabilized, it is necessary to install beam axis adjusting means including various electromagnets in an irradiation device disposed near a patient or in a beam transport path extending to the irradiation device. However, in the broad-beam irradiation method, since the scattering substances are used, a slight variation of a beam axis has a relatively small influence and thus high-accuracy beam axis correcting means is not necessary. In the scanning irradiation method, since a variation of a beam axis in the beam transport system has an influence on an irradiation field to an affected part, high-accuracy beam axis correcting means is necessary.
Accordingly, as the beam axis correcting method in the scanning irradiation method, for example, a correction method in which a scanning electromagnet and a beam position detector are disposed in an irradiation device, a beam position at a target irradiation location is calculated on the basis of a signal from the beam position detector, and the scanning electromagnet is controlled so as to irradiate the target irradiation location with a scanning beam was proposed in the past (for example, see Patent Literature 1). The scanning electromagnet includes two scanning electromagnets for controlling a beam, which travels in the z direction, independently in the x direction and the y direction, an excitation current based on the signal from the beam position detector is made to flow in the electromagnets, and magnetic fields generated in the respective electromagnets are temporally changed to scan the x direction and the y direction with the beam.
In addition, a method was also proposed in which beam transport means for transporting a charged particle beam emitted from an accelerator to an irradiation device is provided with two beam position detecting means and two steering electromagnets, a displacement is calculated on the basis of detection signals output from the respective beam position detecting means, and excitation currents of the steering electromagnets are controlled on the basis of the displacement (for example, see Patent Literature 2).
However, both methods are directed to implementing a predetermined beam trajectory by feeding back a signal, which is calculated on the basis of the detection signal of the beam position detecting means, as an excitation current of the electromagnet in some way and have a structural problem in that the time delay in a control system due to the feedback affects correction accuracy. When a periodic variation is present in a magnetic field of an electromagnet or high-frequency power in the accelerator at the time of taking out a charged particle beam from the accelerator, a position variation or an angle variation of an emitted beam due to the periodic variation is complicatedly affected and thus it is not possible to suppress the position variation or the angle variation of the emitted beam with satisfactory accuracy using only the conventional beam trajectory adjusting means based on the feedback.    Patent Literature 1: JP-A-2009-000347    Patent Literature 2: JP-A-2003-282300